1. Field of the Invention
The present invention relates to an image quality stabilization method executed on an image forming apparatus.
2. Description of the Related Art
Recently, an electrophotographic type image forming apparatus and an inkjet type image forming apparatus have been used. It is desired that such an image forming apparatus is capable of forming an image having a higher quality than a certain level. As one of causes of degradation of image quality, density unevenness (i.e., banding) in a sheet conveyance direction (sub scanning direction) may occur.
Under such circumstances, Japanese Patent Application Laid-Open No. 2007-108246 discusses a method for solving the problem of banding that may occur in the sub scanning direction. More specifically, the method discussed in Japanese Patent Application Laid-Open No. 2007-108246 previously measures banding that may occur in the sub scanning direction with a period due to an outer diameter of a photosensitive drum in association with a phase of the photosensitive drum. Furthermore, the conventional method stores a result of the measurement on a storage unit as a density pattern information table. In addition, during image forming, the conventional method reads information about the banding according to the phase of the photosensitive drum from the table. In addition, the conventional method corrects the banding occurring with the period due to the outer diameter of the photosensitive drum.
Japanese Patent Application Laid-Open No. 2007-108246 also discusses a method, which is similar to the above-described method, for correcting banding that may occur with a period due to an outer diameter of a development roller.
On the other hand, the degree of scatter of toner and the stability of minute dots (hereinafter collectively referred to as a “density stability” or a “dot reproductivity”) may vary due to an environment variation, such as variation in the temperature or the humidity inside or outside the image forming apparatus or a state of use and an operation state of a toner cartridge (hereinafter simply referred to as a “toner CRG”) or a photosensitive drum, such as the consumption or the degradation thereof.
Due to the above-described causes, even if the type of the cause of the density variation in the sub scanning direction (e.g., unevenness in the rotation speed of the photosensitive drum) is the same, the level of the banding may vary. In the following description, the cause of the density variation in the sub scanning direction may be simply referred to as a “sub scanning direction (SSD) density variation cause”.
Now, an exemplary case will be specifically described in detail below with reference to FIGS. 21A and 21B. In the example illustrated in FIG. 21A, in forming a uniform-density image 606, an SSD density variation cause 607 may occur. As the SSD density variation cause 607 illustrated in FIG. 21B, unevenness of the rotation speed of a photosensitive drum is illustrated.
In a lower portion of the example illustrated in FIG. 21B, banding that may occur due to the unevenness of the rotation speed, which is the SSD density variation cause 607, is illustrated. A curve 608 corresponds to banding that may occur when a new toner CRG is utilized. A curve 609 corresponds to banding that may occur when an old toner CRG is utilized, whose dot reproductivity has been deteriorated due to consumption and degradation thereof.
As described above, even if the same SSD density variation cause has occurred, if image forming is executed under different operation conditions, the amplitude (level) of the banding that may become visible on the image may vary. In some cases, the same problem may occur due to environmental variations.